Carbon capture and storage (CCS) is a means of mitigating the contribution of fossil fuel emissions to global warming, based on capturing carbon dioxide (CO2) from large point sources such as fossil fuel power plants, and storing it away from the atmosphere by different means.
Carbon sequestration is a geoengineering technique for long-term storage of carbon dioxide or other forms of carbon to mitigate global warming. Carbon dioxide is usually captured from the atmosphere through biological, chemical or physical processes. It has been proposed as a way to mitigate accumulation of greenhouse gases in the atmosphere, which are released by burning fossil fuels. Carbon dioxide may be captured as a pure by-product in processes related to petroleum refining or from flue gases from power generation. Carbon dioxide sequestration can then be synonymous with the storage part of carbon capture and storage, which refers to large-scale, permanent artificial capture and sequestration of industrially-produced carbon dioxide using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.
Various scrubbing processes have been proposed to remove carbon dioxide from the air, or from flue gases. These usually involve using a variant of the Kraft process. Scrubbing processes may be based on sodium hydroxide. The carbon dioxide is absorbed into solution, transferred to lime and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of carbon dioxide ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which an electrical voltage is applied across the carbonate solution to release the carbon dioxide. While simpler, the electrical process consumes more energy as it splits water at the same time. It also depends on electricity and so unless the electricity is renewable, the carbon dioxide produced during electricity production has to be taken into account. The early incarnations of air capture used electricity as the energy source and therefore depended on carbon-free sources. A thermal air capture system uses heat that can be generated on-site, reducing the inefficiencies associated with producing electricity, but of course it still needs a source of (carbon-free) heat. Concentrated solar power is an example of such a source.
Some examples of carbon dioxide capture onto a salt are the following. First, carbon dioxide is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite (CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide.
As an alternative to sequestration, the recycling carbon dioxide is likely to offer the most environmentally and financially sustainable response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters in the near to medium term. This is because newly developed technologies, such as Bio CCS Algal Synthesis value captured, pre-smokestack carbon dioxide (such as from a coal fired power station, for example) as a useful feedstock input to the production of oil-rich algae in solar membranes to produce oil for plastics and transport fuel (including aviation fuel) and nutritious stockfeed for farm animal production.
Zeolite based CO2 scrubbers suffer from many of the same disadvantages that have been previously mentioned for the broader category of adsorption based scrubbers. Mainly a substantial decrease in CO2 uptake is achieved even with only minor increases in operating temperature. Furthermore the presence of even a small amount of moisture in the system can also greatly reduce the CO2 uptake capacity for zeolite material. For example, CaX, which is one of the best performing zeolites, can have its CO2 uptake capacity reduced from 2.5 mmol CO2 per gram of absorbent to 0.1 mmol CO2 per gram of absorbent with a H2O concentration change from 1 wt % to 16 wt %. This in combination with decrease in capacity with temperature means that the zeolite scrubbers may only be operated at very mild conditions, which are not found in industrial flue gas streams.
The adsorption capacities of activated carbons behave in a similar way to those of zeolites, with rapidly decreasing capacities with slight temperature increases. An example of this is the decrease in CO2 capacity from 3.2 mmol CO2 per gram of adsorbent to 1.5 mmol CO2 per gram of adsorbent with only a slight increase in temperature, from 288 K to 328 K, using Ajax activate carbon as the test scrubber. Also similar to Zeolites the adsorption capacity is greatly affected by the presence of water in the system. An example of this is the decrease in CO2 uptake from 4 mmol CO2 per gram of adsorbent to 1 mmol CO2 per gram of adsorbent when dry coconut shell carbon was wet. These moisture effects only effect the CO2 uptake capacities at low pressures (<25 bar), however the operating costs to pressurize the activate carbon to above 25 bar will come with a large energy penalty.
Calcium oxide based scrubbers do have a very large capacity for CO2 uptake with one very large drawback. In order for the scrubber to achieve a large loading (˜13.4 mmol CO2 per gram of adsorbent) they have to be operated at very high temperatures, ˜1000 K. Also, the practical use of these as large scale CO2 scrubbers is limited by the rate of chemical reaction. This rate is usually very high at massively elevated temperatures (˜1000 K). This additional heating will also incur a very large energy penalty on the system. Furthermore, calcium oxide based scrubber systems suffer from rapid degradation of CO2 uptake capacity during repeated cycling of the system, this is mainly due to pore blocking and adsorbent sintering.
Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkanolamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and carbon dioxide (CO2) from gases. It is a common unit process used in refineries, petrochemical plants, natural gas processing plants and other industries. Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. A typical amine gas treating process includes an absorber unit and regenerator unit as well as accessory equipment. In the absorber, the downflowing amine solution absorbs H2S and CO2 from the upflowing sour gas to produce a sweetened gas stream (i.e., an H2S-free gas) as a product and an amine solution rich in the absorbed acid gases. The resultant “rich” amine is then routed into the regenerator (a stripper with a reboiler) to produce regenerated or “lean” amine that is recycled for reuse in the absorber.
Monoethanolamine (MEA) is the current industrial standard for carbon dioxide capture from industrial flue gas streams despite several disadvantages to the system. The temperature manipulation required for the regeneration of the scrubber system is responsible for up to 70-80% of the operating cost of the scrubber system. Furthermore, as it is a solvent based system, the solvent containing the MEA has to be constantly pumped from one tower to another for absorption and regeneration. The solvent used also has to be constantly replaced due to solvent loss, as the system has to be heated to high temperatures ˜120° C. to regenerate the scrubber. The solvent is also degraded by several other factors including; the high temperature of the flue gas stream itself which is over 100° C. above the ideal temperature for MEA. (See D. Aaron and C. Tsouris, Separation of CO2 from Flue Gas: A Review, Separation Science and Technology, 40:1).
Several approaches have been made to improve the amine process. For example, designing ionic liquids with greater selectivity (E. D. Bates, R. D. Mayton, I. Ntai, and J. H. Davis, Jr., “CO2 capture by a task-specific ionic liquid”, J. Am. Chem. Soc., 2002, 124, 926).
The separation of carbon dioxide from industrial scale coal fired power plants has garnered a lot of attention as the debate regarding climate change intensifies.